Airbus is really excited about an open-rotor engine. Boeing not so much. Here's why.
Don't miss a single episode of the award-winning Check 6. Subscribe in Apple Podcasts, Spotify or wherever you listen to podcasts.
Discover all of our podcasts on our Apple Podcasts channel or at aviationweek.com/podcasts.
Transcript
Joe Anselmo:
Welcome to Aviation Week's Check 6 Podcast. I'm Joe Anselmo, Editorial Director and Editor-in-Chief of Aviation Week magazine.
Boeing is quietly questioning the use of CFM International's proposed open-fan engine to power its next civil airliner. Airbus likes the idea of an open rotor, but Boeing instead appears to be leaning toward advanced ducted engines for whatever replaces the 737. That is potentially troubling news for RISE, a high-profile CFM venture. RISE was undertaken by GE Aerospace and Safran to achieve an ambitious 20% improvement in fuel burn. Aviation Week's Guy Norris, who broke the story of Boeing's skepticism, is with us to explain what it all means. He's joined by Jens Flottau, who runs our global team of commercial aviation editors.
Meanwhile, Pratt & Whitney is now a year into durability problems with its PW1000G geared turbofan engines. Those issues have forced the temporary groundings of hundreds of Airbus A320s, A220s and Embraer E190s. Have the unscheduled inspections now peaked as Pratt is claiming? Are efforts to expand MRO capacity and increase spare parts production, lowering turnaround times, and how much longer will the groundings go on? Editor Sean Broderick, who has written extensively about this, rounds out today's Check 6 panel.
Guy, let's start with you, and maybe you can give our listeners a quick tutorial. Tell us what RISE is and why Boeing is so important to its success.
Guy Norris:
Thanks Joe. I think it's important to go back to basics on what RISE is and what it isn't. CFM wants to really change the game when it comes to engine efficiency, and to do this, it's launched this advanced technology program, RISE, which covers an entire suite of advances. The main, or at least the most obvious part of this suite is this open fan, or open rotor, which is a concept that was explored initially as far back as the 1970s and '80s. And the basic idea behind this is that you have to improve the propulsive efficiency of an engine, and you do this really by moving a lot of air slowly. So if you imagine, in an early jet engine you moved a small amount of air very quickly, but if you can increase the bypass ratio, which is the amount of air that goes around the core relative to the amount of air that goes through the middle of the engine, you can actually maximize the mass flow of air that goes through the engine and is obviously converted into thrust.
And the slower you do this, conversely, it seems sort of counterintuitive in a way, the slower that you can actually, relative to the velocity of the jet flow through the engine, the more efficient it is. So, just to sort of say that another way, you can maximize the propulsive efficiency if the exhaust gas velocity is as close as it can be to the aircraft itself. So, it's more energy efficient to accelerate, therefore, this large amount of air by the small amount than it is to accelerate a small amount of air by a large amount, and you can do that by increasing the size of fan blades.
We've seen the way that the industry has evolved over the past 50 or so years in that sense, the use of high-bypass ratio engines on widebodies, and then of course CFM pioneered the use of high bypass on single-aisle aircraft when it developed the CFM56 in the 1970s. It's worked tremendously well, and of course, the advantages have been huge. Because you've actually increased this propulsive efficiency dramatically by increasing the bypass ratio, you get to this point where you obviously reduce the fuel burn, and that's what people want in the 21st century.
So where do you go from here? Well, the idea behind RISE is that this bypass ratio is contained and constrained at the moment by nacelles. Everybody knows that. You've seen these massive, the GE9X, for example, the 777X, the UltraFan that Rolls Royce is developing, or the Trent XWB on the A350. These are all examples of where the state of the art is now. You've got these giant fans contained within nacelles, but the bigger the nacelle, the heavier they get. They make up to about a third of the weight of the overall engine when you include the reverse thrust system, for example, and the containment which you wrap around that fan system in case of a blade detaching.
The RISE idea is to get away with basically eliminating the restriction of this nacelle and expand the fan diameter accordingly. And if you do this, you immediately get rid of the weight of that containment system, and you maximize the potential efficiency benefit because your bypass ratio can go from, say, between 5 and 10 on the current generation of engines to between 40 and 60, potentially, because you're not constrained by the size of the nacelle, which also is a huge drag maker, by the way, and that's another limiting factor.
Now, the other important aspect to think about it is that the efficiency of the engine is also measured by really the fan pressure ratio. Actually, it tells us the pressure, basically a parameter, it tells us that the acceleration that's put on the air as it passes the fan is measured by this fan pressure ratio. The lower the fan pressure ratio, the higher the propulsive efficiency. Now, GE and Pratt & Whitney looked at this in the '70s and '80s. By the time it was almost ready for showtime in the '90s, fuel prices, which had triggered the panic and the technology push to more efficiency, had gone down again, the technology really frankly wasn't mature at the time, so it was all put on the shelf and kind of forgotten about.
Now, fast-forward another 30 years, the situation's different again. It's an urgent requirement. The industry desperately needs higher efficiency. The entire propulsion industry is targeting these improvements, and lo and behold, the open fan once again is back in the picture, because simply speaking, there is no better way to improve the efficiency to the levels that's required other than going to this idea. The difference with the RISE compared with earlier attempts, including, for example, Safran in France, really pushed this under European Aviation's Clean Sky Program, the research program, in the 2017 period with the CROR program that was basically a modern version of the unducted fan, had two counter-rotating stages facing aft. They tested that under Clean Sky, and it showed massive potential, but it still had problems, still had issues.
Fast-forward to now, CFM is looking at doing this with the RISE but with a different approach. Instead of a pusher, which the Safran engine was, the open fan is a puller, a sort of a tractor engine, a bit like a turboprop. It looks like a turboprop. And instead of having two stages, which introduces all sorts of complications in terms of the hydromechanical controls, the heat involved, and also the complex packaging, they've replaced the stage with effectively a static stator stage. And the importance about this is that it's a pitch adjustable set of swirl recovery veins. It's a much more effective way of countering the swirl losses, which you get from the first stage, and it also avoids the weight complexity and the noise generation issues that you get with the second rotor. So that kind of takes you up to speed. That really shows where CFM is going with this.
Joe Anselmo:
So Guy, you're selling me on this, okay? I'm buying it. Why isn't Boeing buying it? Why does Boeing have doubts?
Guy Norris:
It's a great question. Okay. So, what you really lead me to look at is those are all the positives of the concept. What Boeing is sort of quietly telling us is that there's certain aspects of this installation which you have to counteract, and it's these steps, which are suddenly beginning to counteract really the efficiency benefits that they're calculating you get. So remember, the RISE effort as a whole is sort of targeting a massive generational leap, 20%-plus of fuel efficiency improvement, and that's what's got everybody excited, of course.
But Boeing's basically saying, "Yeah, it's great. Let's do that, and we believe that you can do it for less noise and the technology is there, but what about when you actually have to mount it onto an airframe?" So in the last decade, Airbus, for example, was going to flight test the Safran system, which was aft-facing, and decided at the end that it needed about a ton-and-a-half of extra structural protection just in case one of the blades was released, like protecting an aircraft today from a blade release from a turbofan. Obviously, those turbofans are designed to contain a blade release within that heavy nacelle that we were talking about. You don't have that with an open fan, of course.
CFM, of course, have said, "Well, no, we're dealing with that situation. We are going to have just a single stage rotating, so it's going to be more like today's turboprops, and people fly in turboprops, of course, all the time. And of course, we also can bring 21st technology to that, those blades, frangible-type tips and that sort of thing." Anyway, Boeing's saying, "Yes, that's all very well, but you still will for certification need to protect that structure, protect the fuselage. Whether you're mounting it by a tail or under a wing, it's still going to need protecting."
But what's probably more interesting from their analysis is what happens when you have to put this large 35,000-ish pound of thrust engine on the wing, for example? It's what happens with the installation impacts of that in terms of the loads produced by the rotor during maneuvering and how that translates into the structural strengthening that you'll need into the way that it's installed on the wing. And it's also the airflow effects of that giant open rotor. The CFM RISE, at the moment, is baselined at around 13 feet diameter, which is why it's more likely to be under a wing that's heavily gulled, like a seagull wing, which is what Airbus is looking at, or under a high wing, which is kind of where Boeing is looking at with, say, the X-66. So it gives you enough room, a bit sort of ground clearance for a huge rotor diameter like that.
But it's what happens with the flow of that from that huge rotor as it goes over the wing, and this is nothing new in terms of aerodynamics either. For example, the A400M, the Airbus military transport, they completely saw this happening where their prop fan that they use on that. The effect is that the air flowing over the wing acts like a blown wing so that you get this extra lift from. It's going to be great in terms of field performance. You're going to get short takeoff and landing almost capability, much greater lift. But the downside is that swept aerodynamics over the wing, that swept flow goes in one direction or the other.
So to counter it, you have to have the entire rotation of the prop system going left and right depending on which wing it is, and that means that you cannot easily interchange an engine, in other words. You always have to have right engines for right-hand wings, left engines for left wings, and that's a complication. If you're an airline and you're looking for an engine change, what if you've only got a left engine and you need a right? It's that kind of thing.
So the upshot of all of that is that Boeing is looking at all of these downsides and saying, "Well, if you add it all up, does it really make sense? We could have advanced ducted engines using much of the technology that RISE is going to use, but just keep it within a nacelle, and we think that makes more sense overall." That's the way it's going right now. Of course, it's early days still.
Joe Anselmo:
Jens Flottau, what are the business implications of all this?
Jens Flottau:
It's fascinating scenarios that we're looking at. If an aircraft OEM went for the RISE, it would likely be a single-source solution, because if not, then the other engine OEM would have to build an engine that could be fitted under the same wing as the RISE, which is what's being done on the Airbus side right now with the CFM engine and the Pratt engine. I'm looking at this from a market share point of view, and if you look at the situation from which Boeing has to launch its next generation single-aisle aircraft, it'll be from a position of weakness in terms of market share. Airbus, as we all know, is dominating the single-aisle market. It's almost a two-thirds/one-third split right now.
So what Boeing has to look at when it launches its next aircraft is it has to be as attractive as possible to as broad a customer base as possible. So that tells me dual source. Give the airlines as many options as they can possibly get, attract the Pratt operators, attract the CFM operators, of course, but if you want to do a dual option, then it's obviously much easier to go for a conventional design, a ducted fan that all the engine OEMs have experience in so far or have had in the past decades. Now for Rolls, I guess that applies to Rolls as well if they want to come back into the single-aisle market after having left it.
Now, interestingly, on the Airbus side, things look a little different. Airbus has this big market share, arguably with two engines, two different engine types on the A320neo family, but I could see them more easily go for a single-engine solution, a RISE, for example, because they have this huge market share, they have this very loyal customer base, and also, they seem to be a lot more enthusiastic about RISE than Boeing is.
Just as a reminder, they're planning flight-testing prototypes of the engine on the A380 in a few years. There's tons of meetings going on between CFM people and Airbus people. At the recent Farnborough Airshow, I was visiting the Safran Chalet and was talking to Safran people, and they were telling me all about the issues, some of the issues that Guy just talked about. What happens if a blade breaks? They were talking about technology that would make sure that it fractures at a certain point. So the energy that the fuselage would be exposed to would be lower, would be smaller, so the damage wouldn't be as great. So we're looking at this fascinating scenario where the OEM that has engine choice goes single source, and the other one, Boeing, who only has one engine OEM right now, goes for two. And obviously the question in the next few years is if that happens, who are the two?
Joe Anselmo:
Jens, the baseline for this is that we're assuming that Airbus and Boeing are both going to launch new aircraft, a replacement for the A320 and a replacement for the 737, around the end of this decade, about five years from now or so?
Jens Flottau:
Yes, and as we've reported, Airbus has been putting on the brakes a little bit. They've talked about a launch '27, '28 a year ago, and now it seems like the turn of the decade. So it's all being slowed down by the current issues that we're all seeing in the supply base and the engine and among the engine OEMs in particular. But yes, broadly speaking, turn of the decade.
Guy Norris:
I think the other thing that's worth pointing out is there's a couple more wrinkles, which makes it even more fascinating here. The other one is, and you've briefly alluded to this as well, Jens, CFM also points out that everything that's going into the RISE doesn't necessarily have to end up with an open fan. They're putting a new small high-efficiency core into this. It's a thermodynamically improved small core. It's going to have advanced materials. It's also going to have the potential to be hybrid, mildly hybridized with a sort of hybrid electric element to it. So there's all these new elements.
The other thing, of course, is they need a big powerful gear system to handle this rotor. Instead of powering an open fan, what if you did a large ducted fan again with it within a nacelle? These are all within the suite of technologies which RISE encompasses. So in other words, it may not just be an open fan that we're looking at here. CFM could have and will have the option to produce a new son of LEAP, as it were.
And then added to that, the other element, so what does Pratt and Rolls do about this? Pratt's already very actively looking at doing an advanced next-generation GTF geared Turbofan, and it's got its foot in the door with Boeing already, because it's supplying the engine for its X66 demonstrator, which may just be a couple of engines right now, but strategically it's got massive implications. If they want to be friends with their pals in Seattle, again, could rebuild bridges that have long been washed away by the CFM torrent on single aisles.
And then finally, Rolls put its marker down 10 years ago when it launched the UltraFan program. It successfully demonstrated at large scale that technology just last year. Of course, there is no actual application, a bit like the CFM when the CFM56 when that was originally launched. But they believe that they've got to make a head start with this technology to be mature in time for when the market does come round. But just last year, the new CEO of Rolls laid out this new strategy that they could do something with this geared turbofan technology, and he directed the company to get back into the single-aisle market.
So, as part of what we are hearing, Jens and I at Farnborough, we heard that Boeing is open to talks with Rolls and Pratt about ducted engines. And as Jens mentioned, it's ironic. Boeing set the standard with a single-aisle, one-engine option, one-engine-fits-all kind of policy from 1985 onwards, and they're the ones that are now looking more at the Airbus model for the way to go forward.
Joe Anselmo:
Let's put aside the future for a moment to get to the present, because Sean Broderick's been waiting very patiently here. Sean, this GTF problem has bedeviled Pratt & Whitney for the last year. Are we seeing some light at the end of the tunnel? Have they turned the corner? Are things getting better?
Sean Broderick:
I think saying the last year is being a little generous, Joe, but I get where you're coming from. So the last year has been dominated by the powdered metal inspection, the special fleet management program, and a couple of significant numbers were laid out roughly a year ago. Pratt said that the number of geared turbofan-powered airplanes, again, that's mostly A320s, some A220s and some E2s,. The peak groundings would be around 650 and would happen sometime in the first quarter of 2024, and then they would begin to trend down and settle in at the nice round number of about 350 through the end of 2026. So lots of airplanes.
The other key number was the turnaround time on the overhauls, 300 days wing to wing, 250 to 300 is what Pratt had said. And if you do the math, if the engine started coming off under mandates around last September, the first trickle of the first 100 or so engines that needed to come off, start and then it accelerated through the end of 2023 and into 2024. So 300 days from last September-October is right around now. So those are when the first engines started to come back, right? So we've only begun to see in the last 60 days whether Pratt's turnaround time forecast is accurate and, therefore, whether the rest of its forecast will hold.
The good news is that it seems to be. The 300-day wing-to-wing turnaround time, which is roughly split 150 days in the shops and 150 days in a parking lot waiting for a slot in an overhaul center, that is holding up. The peak number of groundings did occur sometime a little later than the first quarter because of the timing of some of the mandates, was sometime in April or May, did not quite hit 650. Right now, they are sitting in probably in the high 500s. Dan, the numbers guy, gave us numbers of total grounded gear, turbofan-powered airplanes was around 640 on August 1st, and that includes groundings for everything.
And as I alluded to the beginning of this, they have more than just the powdered metal inspection problems going on. They have durability issues dating back really since the debut of the engine. The overhaul shops were basically full when this whole thing started a year ago. One of the reasons why we have 150-day wait just to get a slot in an overhaul shop. So the 640 airplanes on the ground, not all powdered metal-related inspections, most of them are, though. The number's somewhere between 550 and 600. Pratt is not breaking down the specifics of its groundings, but they have not pushed back on our theory that it's in somewhere in the mid to high 500s right now. Trending down, going to hit 350, and it's going to stay there.
Joe Anselmo:
Okay, and Sean, I stand corrected. Pratt is much more than a year into this problem.
Sean Broderick:
It's a little complicated.
Joe Anselmo:
They're a year into implementing their fix for the problem.
Sean Broderick:
It's not as complicated as the RISE stuff that I hope we're not getting quizzed on, but yes, the current year turbofan stuff is fairly complicated. The good news is for operators. It appears as though they can bank on what Pratt is saying, and also, those turnaround times are going to come down as capacity increases. Pratt has added about six shops capable of handling geared turbofan work since this all started. They're up to about 18. It's going to be 19 by the end of the year. Not all of those can do this work, because it requires some special tooling, special inspection tool. But any shop that can do geared turbofan work is going to alleviate the load, because again, there was a lot of durability-related issues that didn't have anything to do with the powdered metal inspections that still needed to be done. So again, they're getting a handle on it. It's a significant issue, but at least now, it's one that's relatively predictable in terms of downtime and duration.
Joe Anselmo:
Well, unfortunately, that is about all the time we have for today's podcast. But thanks, Sean. Thanks to you, Guy, and to you, Jens. And special thanks to our podcast editor in London, Guy Ferneyhough.
One last note to our listeners, executives from across the aircraft engine industry will be convening in Amsterdam on September 11th and 12th for Aviation Week's Aero-Engines Europe Conference. This event will focus on trends in the region's aero-engine community. Aviation Week Editors, Lindsay Bjerregaard and James Pozzi will be onsite, along with the numbers guy, who Sean talked about, Fleet Flight and Data Forecaster, Daniel Williams. Get additional details at aviationweek.com/events and use the promo code CHECK6 to save 20% on registration. That is all the time we have for today. Thank you to our listeners and join us again next week for another Check 6.
Related Content
